Value Adding to Red Grape Pomace Exploiting Eco-friendly FT-NIR Spectroscopy Technique

Agro-food industries require sustainable and profitable alternatives to disposal of their by-products. Grape pomace is a winemaking residue that represents a low-cost natural source of phenolic compounds with recognized antioxidant properties. In this work, Fourier-transform near-infrared (FT-NIR) spectroscopy and chemometric analysis were exploited to the characterization of red grape pomace composition (content of seeds and skins) and chemical properties as total phenolic content (TPC) and total antioxidant capacity (TAC). Raw (n = 96) and milled (n = 96) samples were evaluated by NIR spectroscopy and by classical batchwise assays, Folin-Ciocalteu, and ABTS for TPC and TAC, respectively, after different storage times (1 week to 2 months). Grape seeds had higher levels of TPC and TAC per sample dry weight when compared to grape skins. FT-NIR spectra of raw and milled samples were calibrated against content (%) of skins and seeds, TPC, and TAC using partial least squares (PLS) modeling. Spectral wavelength selection and latent variables were optimized for the lowest root-mean-square errors. PLS models' results showed higher linearity for milled samples (0.936 > R (2) > 0.914) when compared to raw samples (0.885 > R (2) > 0.928). The range error ratio (RER) was between 10 and 14 for raw samples, while for milled grape pomace, it ranged from 15 to 18. Results confirmed that NIR spectroscopy can be applied to winemaking residues with virtually no sample processing needed to estimate the content of grape seeds and skins, the total phenolics, and total antioxidant capacity. Therefore, FT-NIR technique represents a non-destructive and eco-friendly technique to foster added value of grape pomace residues before time-consuming extraction steps are performed.